Radiation has long been used intraoperatively to treat a variety of cancers by delivering a high local dose of radiation directly to a tumor through the operative site. Early intraoperative radiation treatment methods utilized X-rays as the radiation source. More recent intraoperative therapy installations have employed beams of high energy electrons as the radiation source to provide a homogeneous dose of radiation with a rapid falloff in radiation intensity beyond the treatment volume, thereby minimizing exposure of noncancerous tissue to the radiation.
In a typical intraoperative electron beam therapy procedure, the surgeon removes the bulk of patient's tumor so that minimal residual disease remains. The attending radiation oncologist selects the electron beam energy and field size required to treat the target volume. A single radiation dose is then delivered to the tumor site, while the dose delivered to normal tissues is kept to a minimum. An example of an intraoperative electron beam therapy system is disclosed in U.S. Pat. No. 5,321,271 issued Jun. 14, 1994 to Schonberg et al.
During intraoperative treatment of cancer using electron beams, special tubes called applicators are used to shape and guide the electron beam to the treatment site inside the patient without allowing the beam to expose healthy tissue. The treatment head which produces the electron beam must be accurately aligned to the applicator to preserve beam symmetry and uniformity.
In one alignment approach known as hard docking, the applicator is attached directly to the treatment head. Hard docking is not favored because the applicator is simultaneously in contact with the patient and the relatively large and heavy treatment head of the treatment system. If the treatment head is accidentally moved, the applicator may injure the patient.
In soft docking techniques, the treatment head does not physically contact the applicator. One prior art soft docking approach uses laser fan beam alignment to a metal rod that is aligned to the axis of the applicator. The base of the rod and the top of the applicator are located on the machine's isocenter. The disk and rod docking scheme requires that the top of the applicator be at the machine's isocenter height and that the centerline of the applicator be in a plane perpendicular to the center of rotation of the treatment machine. Portable radiotherapy machines have no fixed isocenter position, and the patient plane is strictly determined by geometry, making the use of this scheme difficult.
In another prior art soft docking approach, multiple laser dots are aligned to a scribed line at the top of the applicator. The multiple laser dot docking scheme requires that eight dots from four pairs of lasers be made to coalesce into four dots on a circle scribed on the top surface of the applicator. Mutually orthogonal alignment motions produce similar behavior of the dots on the applicator surface. This makes it difficult to judge which motion of the treatment head will achieve alignment.
All of the prior art docking techniques have had one or more disadvantages, including risk of injury to the patient, difficulty in achieving alignment and lack of an interlock which prevents operation of the system until alignment is achieved. Accordingly, there is a need for improved methods and apparatus for docking a medical treatment system to an applicator.